1. Field of the Invention
The present invention relates generally to variable nozzle turbochargers. More particularly, the invention provides a double axle mounting for the variable vanes of a turbocharger and further includes pressure balancing of the axles for minimizing axial forces tending to act on the vane assembly.
2. Description of the Related Art
In a turbocharger it is often desirable to control the flow of exhaust gas into the turbine to improve the efficiency or operational range. Various configurations of variable nozzles have been employed to control the exhaust gas flow. Multiple pivoting vanes annularly positioned around the turbine inlet and commonly controlled to alter the throat area of the passages between the vanes is an approach which has been successfully used in prior turbochargers. Various approaches to this method for implementing a variable nozzle are disclosed in U.S. Pat. No. 4,679,984 to Swihart et al. entitled "Actuation System for Variable Nozzle Turbine" and U.S. Pat. No. 4,804,316 to Fleury entitled "Suspension for the Pivoting Vane Actuation Mechanism of a Variable Nozzle Turbocharger" having a common assignee with the present application.
Use of cantilevered vanes mounted on an axle such as that disclosed in the '316 patent have been successfully employed in various turbochargers for truck and automotive applications. Under certain operating conditions resulting in a combination of reduced nozzle flow area and elevated turbine inlet pressure, the turbine of the turbocharger effectively operates as an impulse turbine wherein the majority of the drop in stage pressure occurs in the nozzle with the turbine rotor operating at substantially atmospheric static pressure. The large differential pressure acting across the nozzle vanes of the conventional pivoting, cantilevered nozzle vanes creates a reactive couple, which, because of the finite span of the vane axle, results in high reactive side forces and friction. Simultaneously, leakage of exhaust gas from the entry into the nozzle through the operating linkage and nozzle ring supporting the vanes creates an axial force component on the vane mounting axles which forces the ends of the nozzle vanes into the turbine casing shroud wall, creating additional friction. The length of the vane exacerbates the created frictional torque by the long moment arm relative to the axle shaft radius. Movement and control of the vane position is only possible by the application of highly non-linear actuation forces and control hysteresis, due to a combination of friction and "stiction", is excessive.
It is therefore desirable to provide a variable nozzle turbocharger design employing multiple pivoting vanes which reduces the reactive couple on the vane support and further eliminates axial loading of the vane support axles.